The present invention is related to the digital formation of images and it concerns a method and device which make it possible to create a border around a subject incrusted on a background, in particular in digital video; the invention also concerns a special effects generator including such a device.
The video signal V, output from a mixing circuit and corresponding to the incrustation of a subject on a background, has the form EQU V=aS+(1-a)F with 0&lt;a&lt;1
where S and F are the video signals of the subject and background respectively before incrustation, and a is a signal referred to as the key signal of the subject.
A coloured border may be added around the subject in order to make it more visible on the background. To obtain this border, it is possible first to create an incrustation on the background, according to a key signal of a border b, which corresponds to the key signal of the subject expanded in every direction; this first incrustation is produced by a video signal EQU b.B+(1-b)F with 0&lt;b&lt;1
where B is the video signal of the colour of the border; the subject is then incrusted on the result, according to the subject's key signal, a; this second incrustation is therefore given by the video signal EQU V.sub.b =a.S+(1-a) [b.B+(1-b) F]
Alternatively, the subject may first be incrusted on the background, by the video signal EQU V=a.S+(1-a) F
then the border only is incrusted on the result, according to the border key, c; this incrustation is produced by the video signal EQU V'.sub.b =c.B+(1-c) [a. S+(1-a) F],
and it should be noted that V.sub.b ' corresponds to V.sub.b except for the edge effects, and that c is equal to b-a.
This outline incrustation effect also makes it possible to incrust the outline on the background, without the subject.
A known bordering method involves determining the border key signal, b, by comparing, for every point i,j of the screen, the subject's key signal corresponding to that point and the subject's key signals corresponding to the points offset by 1 to n lines in the same column j, then by comparing with one another the key signals of the corresponding points offset by 1 to m points. In order to obtain a border around the outside of the subject, touching its edges, it is therefore sufficient to conserve only the maximum value obtained from the n+1 comparisons made, then to compare these maximum values to one another and again conserve only the maximum value; this latter maximum value therefore constitutes the border key signal allocated to the points on the screen having the coordinates i-n/2,j-m/2, where m and n are even numbers.
This known method can be summarized as follows: to obtain a border of n/2 lines and m/2 points around a subject, a search is performed in a rectangular window of (m+1).times.(n+1) points of the key signal of the subject for the maximum value. This value then constitutes the key signal for the point situated in the center of the window.
This known method presents some disadvantages:
when the subject has corners with acute angles, the corresponding angles of the border are horizontally and/or vertically truncated, PA0 the border is not isotropic because the purely vertical or horizontal edges are less extended than the others. For this reason, when the window is square, an edge at an angle of 45 degrees is the square root of two, that is, approximately 1.4 times more extended than a vertical or horizontal edge; therefore a border around a circular subject tends to be squarish and, the bigger the border in relation to the subject, the more this squareness becomes apparent.